1. Field of the Invention
The present invention relates to a closed type centrifugal pump, and specifically to an impeller structure of a closed type centrifugal pump which is optimally applied to a water pump for use in an automotive-engine liquid-cooling system.
2. Description of the Prior Art
As is generally known, two general types of water pumps suitable for automotive-engine liquid-cooling systems are used: one being an open type centrifugal pump with a rotor having a series of flat or curved vanes or blades on its outer-periphery, and the other being a closed type centrifugal pump with a pair of shrouds formed with a series of blades therebetween. One such closed type centrifugal pump has been disclosed in Japanese Utility-Model provisional publication (Jikkai Showa) No. 58-177584. FIG. 5 shows a structure of the centrifugal pump as disclosed in the Japanese Utility-Model provisional publication No. 58-177584.
Referring now to FIG. 5, the prior art centrifugal pump includes a pump housing 1 which defines a pump chamber 2, a pump shaft 3 which is inserted into the pump chamber 2 and has a driven connection with the engine crankshaft (not shown), and an impeller 4 which is rotatably provided in the pump chamber 2. As seen in FIG. 5, the impeller 4 is firmly secured onto one end of the pump shaft 3 by means of a nut 10. The impeller 4 comprises a rear shroud 5 whose inner peripheral boss-like portion 5a is fixedly connected to the end of the pump shaft 3 by the aid of the nut 10, a front shroud 6 which is located in front of the rear shroud 5 in such a manner as to face the inside curved surface of the rear shroud 5, and a series of blades 7 circumferentially equidistantly disposed between these shrouds 5 and 6. As shown in FIG. 5, the rear shroud 5 is often formed with a pressure-balance hole 5b which is provided for reducing a pressure difference between fluid pressures respectively applied to inside and outside wall surfaces of the rear shroud 5, thus reducing undesired thrust acting on the pump shaft 3, and a substantially annular fluid-flow restricting portion 5c which is provided for restricting the outgoing fluid from the pressure-balance hole 5b and around the pump shaft 3 from being directed toward the outer periphery of the impeller 4. Although it is not clearly shown, the respective blades 7 are backwardly curved with respect to the rotational direction of the impeller 4 and arranged in a vortex fashion. The inner (suction side) edge 7a of each blade 7 faces the pump inlet 8, whereas the outer (pressure side) edge 7b of the blade 7 lies flush with the outermost end of the respective shrouds 5 and 6 and faces the pump outlet 9.
With the above-noted arrangement, when the impeller 4 rotates by rotation of the pump shaft 3, the incoming fluid (the coolant) from the pump inlet 8 is thrown outward through a fluid passage defined between the opposing faces of the two adjacent blades 7 and the opposing inside wall surfaces of the two shrouds 5 and 6, by centrifugal force, and thus the pressurized coolant is forced through the pump outlet 9 and into the water jackets of the engine cylinder block or head.
In the above-mentioned conventional closed type centrifugal pump applied to the forced circulation system, as clearly seen in FIG. 5, the inner edge 7a of the blade 7 is slightly inclined in such a manner as to extend from the inside curved wall of the front shroud 6 to the inner peripheral boss-like portion 5a of the rear shroud 5. The shapes of the respective blades 7, namely dimensions and geometry, are identical with each other. That is, the inner edge 7a of the respective blade 7 has the same inclined angle as indicated in FIG. 5. Therefore, in case that the number of the blades 7 of the same shape is increased simply for the purpose of enhancing a pump efficiency, the overall thickness of the inner edges 7a of all blades 7 is proportionally increased depending on the increased number, thus reducing the overall area of the fluid-flow passageway in the vicinity of the pump inlet 8. As is well known as Bernoulli theorem, when an incompressible (inelastic) fluid flows through a narrower fluid passageway, the fluid velocity will be increased at the narrower area, whereas the fluid pressure is reduced. As a result of the reduction of the overall passage area, the pressure of the coolant tends to be lowered at the pump inlet 8, and remarkably reduced particularly just after the pump inlet. In this case, there is a tendency for cavitation to easily occur. Additionally, there is a possibility that the pump efficiency is reduced owing to collision between the incoming coolant and the respective inner blade edges 7a of the increased number.